That doesn't look like a standard demarc to me. Maybe you are in a multifamily dwelling, or maybe I am out of date. The demarcs I am familiar with use an RJ-11 plug on your side to plug into a socket on the telco side. This allows you to test whether a problem is inside wiring or telco by unplugging your whole inside plant and plugging in a known good phone.
Four wires are standard for residential wiring. As PAT notes, only red and green are used for the first line. This allows a second pair for a second line, or a ground connection for grounded ringing (mostly used in old two party lines).
As PAT also notes, getting the telco hooked up across your VOIP service is ungood. The trouble with doing your connection at the demarc is that telco has access to it and may, possibly inadvertently, reconnect themselves. Also, some telcos leave disconnected lines connected to the switch and able to call 911, much like an unassigned cell phone. You might be better off to cut into your house wiring before the first tap and either disconnect the telco there, or move it over to line two, so you could use their 911 service in an emergency.
I don't know how many terminals you intend to bridge onto your Vonage box, but, if it is like the Packet8 DAT310, it may have trouble driving some of them. I can ring two phones just fine, but there doesn't seem to be quite enough talk battery to keep my speakerphone happy. Of course, that may be more the phone's fault for being overly greedy.[TELECOM Digest Editor's Note: What I do here is the Bell System demarc box is on the wall of my house outside with _two_ lines there from telco but I only use one. I have tape around the modular connector of the second, unused line. I have a small PBX unit inside my house, in a closet near my computer area. From the outside demarc, I bring the one working pair there into my house on my own wires, and into the PBX where it becomes 'dial 9' for outgoing local calls. Then I have my Vonage (VOIP) adapter box near the computer with a connection into the broadband cable line. I go from there with my personally owned modular cable to another input on the PBX, where it becomes 'dial 8' for long distance calls. Both lines (Vonage VOIP) and telco also go through a two-line splitter to which I have a caller ID device and an extra loud ringer (in my old age and feeble condition I am also a wee bit hard of hearing these days as any of you who telephone me know when I periodically ask you to repeat yourself. Then I have several pairs running from the PBX back down the cable to the outside and back to the telco demarc box where _everything_ telco related has been disconnected except for the aforementioned one incoming line.
So to make a local call from any extension, it travels down the pair to the demarc, back in to the PBX, and dial 9 sends it back out the cable to the demarc and off to telco. To make a long distance call from any extension it travels down the pair to the demarc, back in to the PBX where dial 8 sends it across the room to the VOIP box and the broadband internet. To call around my house, it travels down the pair to the demarc, back inside to the PBX where dialing 100 through 105 or0 Zero treats the call as needed, ships it back through the cable to the outside demarc where it gets distributed to where it should go. PAT]